Methods and arrangements for channel estimation

ABSTRACT

Some embodiments provide a method for channel estimation in a wireless device. According to the method, the wireless device obtains (1010) an indication that a set of antenna ports, or antenna port types, share at least one channel property. The wireless device then estimates (1020) one or more of the shared channel properties based at least on a first reference signal received from a first antenna port included in the set, or having a type corresponding to one of the types in the set. Furthermore, the wireless device performs (1030) channel estimation based on a second reference signal received from a second antenna port included in the set, or having a type corresponding to one of the types in the set, wherein the channel estimation is performed using at least the estimated channel properties.

TECHNICAL FIELD

The present invention relates to methods and arrangements for improvedchannel estimation.

BACKGROUND

The 3rd Generation Partnership Project (3GPP) is responsible for thestandardization of the Universal Mobile Telecommunication System (UMTS)and Long Term Evolution (LTE). The 3GPP work on LTE is also referred toas Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is atechnology for realizing high-speed packet-based communication that canreach high data rates both in the downlink and in the uplink, and isthought of as a next generation mobile communication system relative toUMTS. In order to support high data rates, LTE allows for a systembandwidth of 20 MHz, or up to 100 Hz when carrier aggregation isemployed. LTE is also able to operate in different frequency bands andcan operate in at least Frequency Division Duplex (FDD) and TimeDivision Duplex (TDD) modes.

LTE uses orthogonal frequency-division multiplexing (OFDM) in thedownlink and discrete-Fourier-transform-spread (DFT-spread) OFDM in theuplink. The basic LTE physical resource can be seen as a time-frequencygrid, as illustrated in FIG. 1, where each resource element correspondsto one subcarrier during one OFDM symbol interval (on a particularantenna port). There is one resource grid per antenna port.

An antenna port is a “virtual” antenna, which is defined by an antennaport-specific reference signal. An antenna port is defined such that thechannel over which a symbol on the antenna port is conveyed can beinferred from the channel over which another symbol on the same antennaport is conveyed. The signal corresponding to an antenna port maypossibly be transmitted by several physical antennas, which may also begeographically distributed. In other words, an antenna port may betransmitted from one or several transmission points. Conversely, onetransmission point may transmit one or several antenna ports. In thefollowing, an antenna port will be interchangeably referred to as an “RSport”.

In the time domain, LTE downlink transmissions are organized into radioframes of 10 ms, each radio frame consisting of ten equally-sizedsubframes of 1 ms as illustrated in FIG. 2. A subframe is divided intotwo slots, each of 0.5 ms time duration.

The resource allocation in LTE is described in terms of resource blocks,where a resource block corresponds to one slot in the time domain and 12contiguous 15 kHz subcarriers in the frequency domain. Twotime-consecutive resource blocks represent a resource block pair, whichcorresponds to the time interval upon which scheduling operates.

Transmissions in LTE are dynamically scheduled in each subframe. Thebase station transmits downlink assignments/uplink grants to certain UEsvia the physical downlink control information (Physical Downlink ControlChannels, PDCCH, and enhanced PDCCH, ePDCCH). The PDCCHs are transmittedin the first OFDM symbol(s) in each subframe and spans more or less thewhole system bandwidth. A UE that has decoded a downlink assignment,carried by a PDCCH, knows which resource elements in the subframecontain data aimed for the UE. Similarly, upon receiving an uplinkgrant, the UE knows which time/frequency resources it should transmitupon. In LTE downlink, data is carried by the physical downlink shareddata link (PDSCH) and in the uplink the corresponding link is referredto as the physical uplink shared channel (PUSCH).

Demodulation of received data requires estimation of the radio channel,which is performed using reference signals (RS). A reference signalcomprises a collection of reference symbols, and these reference symbolsand their position in the time-frequency grid are known to the receiver.In LTE, cell-specific reference signals (CRS) are transmitted in alldownlink subframes. In addition to assisting downlink channelestimation, they are also used for measurements, e.g. mobilitymeasurements, performed by the UEs. As of Release 10, LTE also supportsUE-specific RS aimed for assisting channel estimation for demodulationof the PDSCH, as well as RS for measuring the channel for the purpose ofchannel state information (CSI) feedback from the UE. The latter arereferred to as CSI-RS. CSI-RS are not transmitted in every subframe andthey are generally sparser in time and frequency than RS used fordemodulation. CSI-RS transmissions may occur every 5th, 10th, 20th,40th, or 80th subframe according to an RRC configured periodicityparameter and an RRC configured subframe offset.

FIG. 3 illustrates how the mapping of physical control and data channelsand reference signals may be done on resource elements within a downlinksubframe. In this example, the PDCCHs occupy the first out of threepossible OFDM symbols, so in this particular case the mapping of datacould start already at the second OFDM symbol. Since the CRS are commonto all UEs in the cell, the transmission of CRS cannot be easily adaptedto suit the needs of a particular UE. This is in contrast to UE-specificRS, where each UE has RS of its own placed in the data region of FIG. 3as part of PDSCH.

A UE operating in connected mode may be requested by the base station toreport channel state information (CSI), e.g., reporting a suitable rankindicator (RI), one or more precoding matrix indices (PMIs) and achannel quality indicator (CQI). Other types of CSI are alsoconceivable, including explicit channel feedback and interferencecovariance feedback. The CSI feedback assists the base station inscheduling, including deciding the subframe and RBs for thetransmission, which transmission scheme/precoder to use, and alsoprovides information on a suitable user bit rate for the transmission(link adaptation). A detailed illustration of which resource elementswithin a resource block pair may potentially be occupied by UE-specificRS and CSI-RS is provided in FIG. 4. The CSI-RS utilizes an orthogonalcover code of length two to overlay two antenna ports on two consecutiveREs. As seen, many different CSI-RS pattern are available. For the caseof 2 CSI-RS antenna ports we see that there are 20 different patternswithin a subframe. The corresponding number of patterns is 10 and 5 for4 and 8 CSI-RS antenna ports, respectively. For TDD, some additionalCSI-RS patterns are available.

Improved support for heterogeneous network operations is part of theongoing specification of 3GPP LTE Release-10, and further improvementsare discussed in the context of new features for Release-11. Inheterogeneous networks, a mixture of cells of differently sized andoverlapping coverage areas are deployed. One example of such deploymentsis illustrated in FIG. 5, where pico cells are deployed within thecoverage area of a macro cell. Other examples of low power nodes, alsoreferred to as points, in heterogeneous networks are home base stationsand relays. The aim of deploying low power nodes such as pico basestations within the macro coverage area is to improve system capacity bymeans of cell splitting gains as well as to provide users with wide areaexperience of very high speed data access throughout the network.Heterogeneous deployments are in particular effective for coveringtraffic hotspots, i.e., small geographical areas with high userdensities served by e.g., pico cells, and they represent an alternativedeployment to denser macro networks.

A classical way of deploying a network is to let differenttransmission/reception points form separate cells. That is, the signalstransmitted from or received at a point is associated with a cell-idthat is different from the cell-id employed for other nearby points.Typically, each point transmits its own unique signals for broadcast(PBCH) and sync signals (PSS, SSS).

The mentioned classical strategy of one cell-id per point is depicted inFIG. 6 for a heterogeneous deployment where a number of low power (pico)points are placed within the coverage area of a higher power macropoint. Similar principles apply to classical macro-cellular deployments,where all points have similar output power and may be placed in a moreregular fashion than what is the case for a heterogeneous deployment.

An alternative to the classical deployment strategy is to instead letall the UEs within the geographical area outlined by the coverage of thehigh power macro point be served with signals associated with the samecell-id. In other words, from a UE perspective, the received signalsappear to be coming from a single cell. This is illustrated in FIG. 7.Note that only one macro point is shown, other macro points wouldtypically use different cell-ids (corresponding to different cells)unless they are co-located at the same site, corresponding to othersectors of the macro site. In the latter case of several co-locatedmacro points, the same cell-id may be shared across the co-locatedmacro-points and those pico points that correspond to the union of thecoverage areas of the macro points. Sync, BCH and control signals areall transmitted from the high power point while data can be transmittedto a UE also from low power points by using shared data transmissions(PDSCH) relying on UE specific RS. Such an approach has benefits forthose UEs that are capable of receiving the PDSCH based on UE-specificRS. Those UEs that only support CRS for PDSCH (which is likely to atleast include all Release 8/9 UEs for FDD) have to settle for thetransmission from the high power point and thus will not benefit in thedownlink from the deployment of additional low power points.

The single cell-id approach is geared towards situations in which thereis fast backhaul communication between the points associated to the samecell. A typical case would be a base station serving one or more sectorson a macro level as well as having fast fiber connections to remoteradio units (RRUs) playing the role of the other points sharing the samecell-id. Those RRUs could represent low power points with one or moreantennas each. Another example is when all the points have a similarpower class with no single point having more significance than theothers. The base station would then handle the signals from all RRUs ina similar manner.

A clear advantage of the shared cell approach compared with theclassical one is that the typically involved handover procedure betweencells only needs to be invoked on a macro basis. Another importantadvantage is that interference from CRS is greatly reduced since CRSdoes not have to be transmitted from every point. There is also muchgreater flexibility in coordination and scheduling among the points.

The concept of a point is heavily used in conjunction with techniquesfor coordinated multipoint (CoMP). In the present disclosure, a point(also referred to as a “transmission point” and/or a “reception point”)corresponds to a set of antennas covering essentially the samegeographical area in a similar manner. Thus, a point might correspond toone of the sectors at a site, but it may also correspond to a sitehaving one or more antennas all intending to cover a similargeographical area. Often, different points represent different sites.Antennas correspond to different points when they are sufficientlygeographically separated and/or have antenna diagrams pointing insufficiently different directions.

Techniques for CoMP entail introducing dependencies in the scheduling ortransmission/reception among different points, in contrast toconventional cellular systems where a point, from a scheduling point ofview, is operated more or less independently from the other points. Onefundamental property of DL CoMP is the possibility to transmit differentsignals and/or channels from different geographical locations. One ofthe principles guiding the design of the LTE system is transparency ofthe network to the UE. In other words, the UE should be able todemodulate and decode its intended channels without specific knowledgeof scheduling assignments for other UEs or network deployments.

For example, different CSI-RS patterns may be transmitted from portsbelonging to different transmission points. Feedback based on suchpatterns may be exploited e.g. for point selection and/or foroptimization of precoding weights and CoMP scheduling. Alternatively,the same CSI-RS pattern may be jointly transmitted by differenttransmission points in order to generate an aggregated feedbackincluding joint spatial information for multiple points. In any case,UEs are generally not aware of the geographical location from which eachantenna port is transmitted.

CRS are typically transmitted from a static set of points. Nevertheless,for certain deployments, it is possible to transmit different CRS portsfrom different geographical locations. One application of this techniqueis in distributed deployments, where the transmit antennas belonging tothe same node are deployed in a non-collocated fashion.

DMRS or UE-specific RS are employed for demodulation of data channelsand possibly certain control channels (ePDCCH). Data may be transmittedfrom different points than other information (e.g. control signaling).This is one of the main drivers behind the use of UE-specific RS, whichrelieves the UE from having to know many of the properties of thetransmission and thus allows flexible transmission schemes to be usedfrom the network side. This is referred to as transmission transparency(with respect to the UE). A problem is, however, that the estimationaccuracy of UE-specific RS may not be sufficient in some situations.Furthermore, especially in case of CoMP and/or distributed deployments,the DMRS for a specific UE-might be transmitted from geographicallyseparated ports.

There is a need in the art for mechanisms for improved channelestimation.

SUMMARY

An object of some embodiments is to provide a mechanism for improvedchannel estimation, in particular in CoMP scenarios.

Some embodiments provide a method for channel estimation in a wirelessdevice. According to this method, the wireless device obtains anindication that a set of antenna ports, or antenna port types, share atleast one channel property. The device then estimates one or more of theshared channel properties. The estimation is based at least on a firstreference signal received from a first antenna port included in the set,or having a type corresponding to one of the types in the set. Further,the wireless device performing channel estimation based on a secondreference signal received from a second antenna port included in theset, or having a type corresponding to one of the types in the set. Thechannel estimation is performed using at least the estimated channelproperties.

In particular embodiments, the wireless device receives a message fromthe network node comprising the indication. In other embodiments, theset is determined based on a rule.

In some embodiments, the wireless device is thus able to use estimatedchannel properties for one antenna port in the estimation for anotherantenna port, by assuming co-location of certain antenna ports. In aparticular example, the device may perform joint estimation based on twoor more antenna ports, which leads to improved estimation accuracy. Inanother example, the device may apply an estimated property for one portto another port in the set, which may lead to a faster estimationprocess.

Some embodiments provide a method in a network node. The methodcomprises obtaining an indication that a set of antenna ports, orantenna port types, share at least one channel property. The networkthen transmitting signals corresponding to at least two of the antennaports in the set, or antenna ports having types comprised in the set,from the same set of transmission points.

Thus, in some embodiments the network node enables the wireless deviceto perform improved channel estimation, by ensuring that certain antennaports are co-located.

Some embodiments provide a wireless device for performing channelestimation. The device comprises radio circuitry and processingcircuitry. The processing circuitry further comprises a channel analyzerand a channel estimator. The processing circuitry is configured toobtain an indication that a set of antenna ports, or antenna port types,share at least one channel property. The channel analyzer is configuredto estimate one or more of the shared channel properties based at leaston a first reference signal transmitted from a first antenna portincluded in the set, or having a type corresponding to one of the typesin the set, wherein the first reference signal is received via the radiocircuitry. The channel estimator is configured to perform channelestimation based on a second reference signal transmitted from a secondantenna port included in the set, or having a type corresponding to oneof the types in the set, wherein the channel estimation is performedusing at least the estimated channel properties, and wherein the secondreference signal is received via the radio circuitry.

Some embodiments provide a network node comprising radio circuitry andprocessing circuitry. The processing circuitry is configured to obtainan indication that a set of antenna ports, or antenna port types, shareat least one channel property. The processing circuitry is furtherconfigured to transmit, via the radio circuitry, at least two of theantenna ports in the set, or antenna ports having types comprised in theset, from the same transmission point;

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the LTE downlink physicalresource.

FIG. 2 is a schematic diagram showing the LTE time-domain structure.

FIG. 3 is a schematic diagram showing an LTE downlink subframe.

FIG. 4 is a schematic diagram showing possible reference signalpatterns.

FIG. 5 is a schematic diagram showing an example macro and pico celldeployment.

FIG. 6 is a schematic diagram showing an example heterogeneousdeployment.

FIG. 7 is a schematic diagram showing another example heterogeneousdeployment.

FIG. 8 is a schematic diagram illustrating a wireless communicationsystem.

FIG. 9 is a flow chart showing an example method in a network node.

FIG. 10 is a flow chart showing an example method in a wireless device.

FIG. 11 is a flow chart showing an example method in a network node.

FIG. 12 is a flow chart showing an example method in a wireless device.

FIG. 13 is a flow chart showing an example method in a network node.

FIG. 14 is schematic diagram showing an example network.

FIG. 15 is a block diagram showing an example network node.

FIG. 16 is a block diagram showing an example wireless device.

DETAILED DESCRIPTION

As explained above, reference signals may be transmitted fromgeographically separated ports. Geographical separation of RS portsimplies that instantaneous channel coefficients from each port towardsthe UE are in general different. Furthermore, even the statisticalproperties of the channels for different ports and RS types may besignificantly different. Example of such statistical properties includethe SNR for each port, the delay spread, the Doppler spread, thereceived timing (i.e., the timing of the first significant channel tap),and the number of significant channel taps. In LTE, nothing can beassumed about the properties of the channel corresponding to an antennaport based on the properties of the channel of another antenna port.This is in fact a key part of maintaining transmission transparency.

Based on the above observations, the UE needs to perform independentestimation for each RS port of interest for each RS. This may result ininadequate channel estimation quality for certain RS ports, leading toundesirable link and system performance degradation. A related problemwhich indirectly also affects the estimation accuracy is that it is notpossible for the UEs to assume co-location of DMRS ports with other RSports, particularly in CoMP scenarios.

Some embodiments disclosed herein provide the UE with selectedinformation about RS ports grouping, in order to allow channel estimatorimplementations to exploit common channel properties for different RSports and/or RS types within a group. The information comprises e.g. ofsignaling which reference signals may be assumed to be used incombination with each other to form a channel estimate corresponding toa certain antenna port. Similarly but stated differently, which antennaports may be assumed to have channels that can be utilized for inferringproperties of the channel over which symbols for the antenna port ofinterest is conveyed. That is, the UE may be signaled that it is allowedto assume that reference signals on some antenna ports may be used toassist in the channel estimation of a channel for another antenna port.

The antenna ports whose channels exhibit such mutual dependence can besaid to form a group. In practice, this assumption would allow the UE toassume that at least some statistical properties of the channels aresimilar over different antenna ports. Such information allows the UE tojointly estimate channel properties and to achieve increased estimationaccuracy for the corresponding channels estimates. Thus, particularembodiments enable improved channel estimation by enabling joint channelparameters when it is applicable.

The network typically configures the UE to assist reception of varioussignals and/or channels based on different types of reference signalsincluding, e.g., CRS, DMRS, CSI-RS. Possibly, RS may be exploited forestimation of propagation parameters and preferred transmissionproperties to be reported by the UEs to the network, e.g., for linkadaptation and scheduling.

It is observed here that, even though in general the channel from eachantenna port to each UE receive port is substantially unique, somestatistical properties and propagation parameters may be common orsimilar among different antenna port, depending on whether the differentantenna ports originate from the same point or not. Such propertiesinclude, e.g., the SNR level for each port, the delay spread, theDoppler spread, the received timing (i.e., the timing of the firstsignificant channel tap) and the number of significant channel taps.

Typically, channel estimation algorithms perform a three step operation.A first step consists of the estimation of some of the statisticalproperties of the channel. A second step consists of generating anestimation filter based on such parameters. A third step consists ofapplying the estimation filter to the received signal in order to obtainchannel estimates. The filter may be equivalently applied in the time orfrequency domain. Some channel estimator implementations may not bebased on the three steps method described above, but still exploit thesame principles.

Obviously, accurate estimation of the filter parameters in the firststep leads to improved channel estimation. Even though it is often inprinciple possible for the UE to obtain such filter parameters fromobservation of the channel over a single subframe and for one RS port,it is usually possible for the UE to improve the filter parametersestimation accuracy by combining measurements associated with differentantenna ports (i.e., different RS transmissions) sharing similarstatistical properties. It is observed here that the network istypically aware of which RS ports are associated with channels withsimilar properties, based on its knowledge of how antenna ports aremapped to physical points, while the UE is not aware a-priori of suchinformation because of the network transparency principle.

Some embodiments comprise including information in the downlinksignaling in order to enable improved channel estimation in the UE. Morespecifically, the signaling from the network to the UE includesinformation about which reference signals and/or antenna ports and/or RStypes may be assumed by the UE to be used together when demodulatingsignals on certain antenna port(s), i.e., which antenna ports can beassumed to share similar channel properties. The UE may then exploitsuch information to perform joint or partly joint channel estimation forat least some of the channels with similar properties.

Often, channels associated to RS transmitted from the same point sharesimilar statistical properties and propagation characteristics.Therefore, in one example, the network may indicate by signaling to theUE at least a subset of the RS that are transmitted from a common point.

Signaling to the UE of the RS ports associated with similar channelproperties may be defined in different ways. In one example, the networkincludes in the semi-static RRC signaling an index for at least some ofthe RS ports and types sharing similar properties. RS ports associatedwith the same index are assumed by the UE to be associated with similarchannel properties.

In another example, a signaling flag is employed to signal if a certainsubset of RS ports is associated to the same channel.

In another example additional bits may be provided to signal if acertain RS type (e.g., CRS, DMRS, CSI-RS) maybe be assumed to sharesimilar channel properties of some of the other channel types. In oneexample, a flag is provided for at least some of the DMRS and/or CSI-RSports. When such flag is enabled, the UE assumes that all thecorresponding RS ports share the similar channel properties as CRS.

In a further example, two flags per subset of RS ports are provided, oneof such flags indicating that all the associated ports share similarchannel properties and the other flag indicating that the associatedports share similar channel properties as at least some of the CRSports.

The flexibility of signaling grouping of at least partly dependentantenna ports can vary and in one embodiment it may be based onsignaling dependence between antenna ports of different types, e.g.,that the channels on the antenna ports where CRS are transmitted may beused to infer properties of the channel on antenna ports over which UEspecific RS are transmitted (e.g., DMRS). Alternatively, CSI-RS may beexploited for assisting in the estimation of channels for antenna portscarrying UE specific RS.

All the signaling examples described above may be alternatively carrieddynamically by scheduling assignments, i.e., DCI formats. In such case,PDCCH should be decoded based on the RS properties indicated by RRCsignaling (if any), while the additional RS properties indicated by thescheduling grants are exploited for PDSCH demodulation. The dynamicsignaling could even include signaling which CSI-RS resource (or CRSincluding information for identifying which cell the CRS is associatedwith) may be assumed by the UE to be allowed to be used in assisting inDMRS/UE specific RS channel estimation in a certain transmission,providing efficient support for dynamic point selection.

Furthermore, the above examples may be combined for further flexibility.

In further examples, the network may base grouping of RS properties on asubset of the channel properties, as well as other criteria.

One possibility is to define that RS ports that are made orthogonal byuse of orthogonal codes (e.g., OCC) always share the similar channelproperties. Such assumption may be statically defined a-priori (i.e.,defined in the standard) or alternatively signaled in a semi-staticfashion by the network.

In another example the network signals to the UE that a subset of RSports shares similar propagation properties when such RS ports areassociated with collocated antennas with the same polarization.

In further examples the network signals to the UE that a limited set ofchannel properties (e.g., timing and/or Doppler and/or delay spreadand/or SNR) are shared by a subset of RS ports. Some of the signalingprocedures described above may be exploited, in addition to thedefinition of which set of channel properties shall be assumed by the UEto be common to the indicated RS ports.

Within the context of this disclosure, the term “shared channelproperty”, “shared channel parameter”, or “shared propagationparameter”, means that the value of a propagation parameter, i.e.channel property, for the channel corresponding to one antenna port isthe same as, or similar to, the value of the same parameter for thechannel corresponding to another antenna port. In this context,“similar” means that the values differ by less than a predefined amount,which may be viewed as a margin of tolerance. Thus, the sharedproperties are either the same, or similar enough that they may be usedfor channel estimation for any of the channels with acceptable accuracy.Stated differently, shared properties are sufficiently similar to allowimproved estimation by jointly estimating them as a common property forthe corresponding RS ports.

In further examples, in order to reduce signaling overhead, grouping ofRS ports is defined a-priori for certain RS ports of the same type. Thisallows UEs to exploit common channel properties for such groups withoutneed of dedicated signaling. In case some of the above signalingtechniques are also employed, it is possible to signal channelproperties commonality between a-priori defined groups of RS ports,instead for each port individually. Such hard-coded dependence mandatesin fact that the corresponding ports are transmitted from the samepoint(s) by the network.

Additionally, some embodiments include the possibility of defining ana-priori timing relation between synchronization signals (e.g., SSSand/or PSS) and some RS-port(s), enabling the UE to infer RS timing fromthe synchronization signals. The UE is thus able to perform joint timingestimation for the synchronization signals and the RS ports with relatedtiming. The timing relation may be expressed, e.g., in terms of apre-defined timing difference range and/or offset.

FIG. 8 illustrates a wireless communications system 800 in which variousembodiments of the invention may be implemented. Three network nodes820, 850 and 870 are shown. Network node 820 control two transmissionpoints 830 and 840. Similarly, network node 850 controls onetransmission point 860, and network node 870 controls transmissionpoints 880 and 890. In the following, for purposes of illustration andnot limitation, it will be assumed that the communications system 800 isan LTE system, and that the network nodes 820, 850 and 870 are eNodeBs.The transmission points may correspond to separate antennas at theeNodeBs, e.g. sector antennas, or they may be remote radio units (RRU:s)connected to the respective eNodeB. The number of transmission pointsshown shall not be construed as limiting; it will be appreciated that,generally speaking, each network node may control one or moretransmission points, which may either be physically co-located with thenetwork node, or geographically distributed. Furthermore, although thisexample shows different transmission points being controlled bydifferent network nodes, another possible scenario is that one singlenetwork node controls several or even all of the transmission points830, 840, 860, 880 and 890. In the scenario shown in FIG. 8, it isassumed that some or all of the network nodes 820, 850, and 870 areconnected, e.g. by means of a transport network, to be able to exchangeinformation for possible coordination of transmission and reception.

The communications system 800 further comprises two wireless devices 810and 812. Within the context of this disclosure, the term “wirelessdevice” encompasses any type of wireless node which is able tocommunicate with a network node, such as a base station, or with anotherwireless device by transmitting and/or receiving wireless signals. Thus,the term “wireless device” encompasses, but is not limited to: a userequipment, a mobile terminal, a stationary or mobile wireless device formachine-to-machine communication, an integrated or embedded wirelesscard, an externally plugged in wireless card, a dongle etc. The wirelessdevice may also be a network node, e.g. a base station.

With reference to FIG. 8 and the flowchart in FIG. 9, a method in anetwork node according to some embodiments will now be described. Themethod may be executed in one of the network nodes 820, 850, or 870.

Throughout this disclosure, whenever it is stated that two antenna portsare co-located, or can be assumed to be co-located, this means, in ageneral sense, that at least one of the following channel parameters,i.e. propagation properties:

-   -   Delay spread    -   Doppler spread    -   Signal to noise ratio    -   Frame synchronization

of the channel over which a symbol on the first antenna port is conveyedcan be inferred from the channel over which another symbol on the otherantenna port is conveyed. As a particular example, a set of antennaports are co-located if their corresponding signals are transmitted fromthe same set of transmission points, where the set may comprise one ormore points. It should be noted in this context that the signal (e.g.RS) corresponding to an antenna port may be transmitted in distributedfashion from multiple points. Two such distributed antenna ports areconsidered to be co-located if the corresponding signals are transmittedfrom the same set of points. For example, referring to FIG. 8, CSI-RS 19is transmitted from points 830 and 840, and CSI-RS 20 is alsotransmitted from 830 and 840. Thus, CSI-RS ports 19 and 20 areco-located. Stated differently, two antenna ports may be considered tobe co-located if their corresponding reference signals are mapped to thesame “virtual point”, where a virtual point is defined as a combinationof one or more points.

According to the method, the network node obtains 910 an indication thata set of antenna ports share at least one channel property. It will beappreciated that this is just a short-hand way of saying that thechannels corresponding to each of the antenna ports in the set share atleast one channel property, i.e. propagation parameter. The indicationmay be obtained in different ways. In one variant, the network nodedetermines the set based on a rule. In another variant, the network nodeobtains the indication either from another network node, or from astorage area, e.g. a memory or a database. The storage area may beinternal to the network node, or connected to it.

As a specific example, the network node obtains an indication that CRS 0and CRS 2, and CRS 1 and 3 should be co-located. Thus, CRS 0 and CRS 2form one set, and CRS 1 and CRS 3 form another set. This is illustratedin FIG. 8 by network node 820, which transmits CRS 0 from point 830, andCRS 1 from point 840. This particular grouping is thus compatible withdistributed antenna deployment for two transmit antennas. It is furthernoted that, since cross-polarized antennas are often employed when twotransmit antennas are used, it may be beneficial to enforce independentchannel estimation for ports 0 and 1. However, if the performance fortwo antennas is a concern, it is equally possible to group the ports CRS0 and CRS 1, and the ports CRS 2 and CRS 3. This would limit distributedCRS deployments to the 4 antenna case.

In another specific example, obtaining the indication comprisesdetermining that antenna ports transmitting reference signals that arecode multiplexed using an orthogonal cover code are part of the sameset. For example, a possible grouping of CSI-RS is (15,16) (17,18)(19,20) (21,22), and a possible grouping of DMRS is (7,8) (9,10) (11,13)(12,14) or (7,8,11,13) (9,10,12,14).

One of the possible CSI-RS groupings is illustrated in FIG. 8 by networknode 820. An advantage of transmitting signals corresponding to antennaports that are divided by an OCC from the same antenna port is that thesignals will have the same Doppler. OCC is ineffective at high Doppler,and orthogonality is destroyed. If we divide high and low Doppler portsby OCC, all slow ports will be interfered by the fast ports. On thecontrary, if OCC-multiplexed ports are grouped by Doppler, the pairedslow ports will preserve orthogonality while only the fast ports,mutually interfering with each other, will lose orthogonality.

In a variant of this embodiment, the indication is based on antenna porttypes, rather than individual antenna ports. That is to say, in step 910the network node obtains an indication that a set of antenna port typesshare at least one channel property. The antenna port types correspondto types of reference signals transmitted by the ports. Some examples ofantenna port types are DMRS, CSI-RS, or CRS. As a particular example,the network node obtains an indication that CRS and DMRS ports should beco-located, i.e. that all CRS and DMRS ports should be transmitted fromthe same transmission point, or set of points. As another example, thenetwork node obtains an indication that ports of type DMRS should beco-located, i.e. the antenna ports that are transmitting DMRS should beco-located. Optionally, the indication may be device-specific. Forexample, the network node may obtain an indication that CRS and DMRSports for a specific device should be co-located. This option isillustrated in FIG. 8 by network node 870, which transmits CRS 0 andDMRS 9-10, directed to wireless device 810, from the single transmissionpoint 890, and transmits CRS 1 and DRMS 11-12, which are directed towireless device 812, from transmission point 880.

Further example groupings, which may be applied in this embodiment, areshown in Table 1 below.

TABLE 1 Examples of pre-defined co-location rules. Example groupings CRSDMRS CSI-RS Example 1 (0, 2), (1, 3) (7, 8), (9, 10), (15, 16), (17,18), (suitable for (11, 13), (12, 14) (19, 20), (21, 22) interleavedindoor deployments) Example 2 (0, 1), (2, 3) (7, 8), (9, 10), (15, 16),(17, 18), (optimized for 2tx (11, 13), (12, 14) (19, 20), (21, 22)non-interleaved deployments)

The network node then transmits 920 signals corresponding to at leasttwo of the antenna ports in the set from the same transmission point orpoints. By transmitting signals for several antenna ports from the samepoint or points, the network node ensures that those antenna ports willbe co-located according to the above definition. This enables a wirelessdevice, e.g. device 810 or 812, to perform joint estimation of theshared channel properties. Alternatively, a UE with limited processingcapabilities may estimate the channel properties for the signalcorresponding to one or more of the antenna ports and exploit suchproperties for estimating the channels corresponding to other co-locatedantenna ports.

In this context, the signals corresponding to an antenna port comprisethe antenna port-specific reference signal, as well as the correspondingdata transmitted on the antenna port.

With reference to FIG. 8 and the flowchart in FIG. 10, a method forchannel estimation in a wireless device according to some embodimentswill now be described. The method may be implemented in one of thewireless devices 810 or 812.

According to the method, the wireless device obtains 1010 an indicationthat a set of antenna ports, or antenna port types, share at least onechannel property. The indication may be obtained in different ways. Inone variant, the wireless device determines the set based on a rule. Inanother variant, the wireless device obtains the indication from aninternal storage area, e.g. a memory or a database.

The indication may indicate that the set of antenna ports or antennaport types can be considered to be co-located. As mentioned above, thisimplies that at least one of the properties delay spread, Dopplerspread, signal-to-noise ratio, and frame synchronization are shared. Inparticular variants, the indication may indicate that all theseproperties are shared. In the case where only some properties areshared, the wireless device may further obtain, from the network node,an indication of which properties are shared. Alternatively, thewireless device could be preconfigured to assume that certain propertiesare shared.

In a particular example, antenna ports whose reference signals are codemultiplexed using an orthogonal cover code are considered to be part ofthe same set, and antenna ports whose reference signals are not codemultiplexed using an orthogonal cover code are considered to be part ofdifferent sets.

Other example port groupings correspond to those described above inconnection with FIG. 9. Notably, when the indication is based on a ruleor obtained from internal storage, it is implied that the network nodeor nodes serving the wireless device follow the same rule, and actuallytransmit the ports that are assumed to be co-located from the samepoint. This may be ensured by encoding the rule or rules into astandard.

The wireless device then jointly estimates 1020 one or more of theshared channel properties, based on a first reference signal receivedfrom a first antenna port included in the set, or having a typecorresponding to one of the types in the set, and on a second referencesignal received from a second antenna port included in the set, orhaving a type corresponding to one of the types in the set. Optionally,the joint estimation may be based on more than two reference signalscorresponding to antenna ports or port types in the set. The channelproperties to be estimated may be one or more of the signal-to-noiseratio, delay spread, doppler spread, received timing, and the number ofsignificant channel taps.

After estimating the shared properties, the wireless device performs1030 channel estimation based on the second reference signal. Thechannel estimation is performed using at least the estimated channelproperties. In particular embodiments, the step of performing channelestimation comprises generating an estimation filter based on theestimated channel properties, and applying the estimation filter to thesecond reference signal to obtain a channel estimate. Optionally, thewireless device may apply the estimation filter to at least one otherreference signal received from an antenna port included in the set, orhaving a type corresponding to one of the types in the set, to obtain asecond channel estimate.

With reference to FIG. 8 and the flow chart in FIG. 11, a method in anetwork node according to some embodiments will now be described. Themethod may be executed in one of the network nodes 820, 850, or 870.

According to the method, the network node obtains 1110 an indicationthat a set of antenna ports, or antenna port types, share at least onechannel property. This step corresponds to 910 above, and the samevariants apply.

The network node then transmits 1120 signals corresponding to at leasttwo of the antenna ports or antenna port types in the set from the sametransmission point or points. This step corresponds to 920 above, andthe same variants apply.

The network node then transmits 1130 an indication of the set of antennaports, or antenna port types, to at least one wireless device served bythe network node, e.g. wireless device 810 or 812. The indication may betransmitted in system information, in an RRC message, or in downlinkcontrol information.

In a variant, the indication further indicates one or more resourceblock groups (RBGs) across which the set of antenna ports or antennaport types share at least one channel property. A resource block group(RBG) consists of a set of consecutive physical resource blocks, whichare not necessarily adjacent to each other. Generally, an RBG maycomprise 1 to 4 resource blocks, depending on the system bandwidth. Thenetwork node may apply a different precoder for each RBG, which impliesthat different channel properties may exist for each RBG. Differenttransmission points may also employed for each resource block group.

As a particular example, the network node may indicate to the wirelessdevice that certain DMRS ports share the same channel properties for agiven set of RBGs in a subframe, and for the given wireless device.

As another example, the network node indicates to the wireless devicethat a set of RBGs share similar channel properties as a given set ofantenna ports, or antenna port types.

In another variant, the indication further indicates a time intervalduring which the set of antenna ports or antenna port types share atleast one channel property. The network node transmits 1120 signalscorresponding to at least two of the antenna ports in the set, orantenna ports having types comprised in the set, from the same set oftransmission points during the indicated time interval. The timeinterval may e.g. be indicated as a number of subframes. This enablesthe wireless device to time-average the corresponding estimated channel.If no time interval is indicated, the network node may transmit theantenna ports in the set from the same set of transmission points duringa predefined time period, or until a new indication is sent. Thisvariant may be used in conjunction with signalling a set of RBGs overwhich the ports share similar channel properties.

In another variant, the network node transmits a further indication of atiming relation between one or more antenna ports, and one or moresynchronization signals, e.g. SSS and/or PSS. This enables the wirelessdevice to infer RS timing from the synchronization signals and toperform joint timing estimation as discussed above. The timing relationmay be indicated, e.g., in terms of a pre-defined timing differencerange and/or offset.

In yet another variant, the indication is provided in the form of amessage configuring the device to report wideband feedback (e.g.,wideband PMI on PUCCH1-1 or PUCCH2-1 or PUCCH3-1). This indicates to thewireless device that the UE-specific DMRS ports are co-located over theentire PDSCH bandwidth. Alternatively, a first indication may indicatethat the DMRS are co-located. Subsequently, the network node configuresthe device to report wideband feedback, and this indicates that theco-location applies over the entire bandwidth.

With reference to FIG. 8 and the flowchart in FIG. 10, a method forchannel estimation in a wireless device according to some embodimentswill now be described.

According to the method, the wireless device receives 1010 a messagefrom a network node, the message comprising an indication that a set ofantenna ports, or antenna port types, share at least one channelproperty. The indication may indicate that the set of antenna ports orantenna port types can be considered to be co-located, i.e. that allchannel properties are shared.

The wireless device then jointly estimates 1020 one or more of theshared channel properties, as has been described above.

After estimating the shared properties, the wireless device performs1030 channel estimation based on the second reference signal, in thesame way as has been described above.

In a variant, the indication further indicates one or more resourceblock groups (RBGs) across which the set of antenna ports or antennaport types share at least one channel property. As a particular example,the wireless device may receive an indication that certain DMRS portsshare the same channel properties for a given set of RBGs in a subframe.In step 1020, the wireless device then estimates the shared propertiesover the given set of RBGs. Across the remaining RBGs, the wirelessdevice performs individual estimation of channel properties.

As another example, the network node indicates to the wireless devicethat a set of RBGs share similar channel properties as a given set ofantenna ports, or antenna port types.

In another variant, the indication further indicates a time intervalduring which the set of antenna ports or antenna port types share atleast one channel property. The channel estimator may then time-averagethe corresponding channel estimates. The time interval may e.g. beindicated as a number of subframes. If no time interval is indicated,the wireless device may assume the channel properties will be sharedduring a predefined time period, or until a new indication is received.This variant may be used in conjunction with receiving an indication ofa set of RBGs over which the ports share similar channel properties.

In another variant, the wireless device receives a further indication ofa timing relation between one or more antenna ports, and one or moresynchronization signals, e.g. SSS and/or PSS. The wireless device infersRS timing from the synchronization signals, and in step 1020 thewireless device also performs joint timing estimation as discussedabove.

In yet another variant, the indication is received in the form of amessage configuring the device to report wideband feedback (e.g.,wideband PMI on PUCCH1-1 or PUCCH2-1 or PUCCH3-1). This indicates to thewireless device that the UE-specific DMRS ports are co-located over theentire PDSCH bandwidth. Alternatively, the wireless device obtains afirst indication that the DMRS are co-located (this indication may beobtained in any of the ways described above). Subsequently, the wirelessdevice receives a message configuring it to report wideband feedback,and this indicates to the wireless device that the co-location appliesover the entire bandwidth.

In the embodiments described in connection with FIGS. 9-11, it has beenassumed that the wireless device applies joint estimation, i.e. performsmeasurements on several reference signals, which are combined to form ajoint estimate. This leads to improved accuracy. However, anotherpossibility is to estimate the shared properties based on one referencesignal, and then apply the estimated properties when performing channelestimation on another reference signal. This may lead to improvedperformance, as fewer measurements need to be performed.

According to some embodiments, a method in a wireless device, e.g., aUE, for performing channel estimation is provided. This method isillustrated in FIG. 12.

The wireless device obtains information indicating that a set ofreference signal (RS) ports share at least one channel property, orchannel parameter.

In a variant, the wireless device receives information from a networknode, e.g., a base station such as an eNB, indicating the set ofreference signal ports. The set may be associated with an index, and theinformation may then comprise an indication that one or more RS portsare associated with the index. Various other ways of representing theinformation are possible, and will be described below. The informationmay be comprised in an RRC message, or in a DCI format.

In another variant, the wireless device determines the set of RS portssharing at least one channel property based on a rule or predeterminedmapping. For example, the wireless device may assume that RS ports thatare code division multiplexed together using orthogonal codes share thesame channel properties.

The wireless device then performs joint estimation of the shared channelproperties for the reference signals corresponding to the ports in theset.

The set of reference signals may be equivalently referred to as a groupof reference signals.

According to some embodiments, a method in a network node, e.g., a basestation such as an eNB, is provided. This method is illustrated in FIG.13.

The network node determines a set of reference signal (RS) ports thatshare at least one channel property, or channel parameter. In oneexample, the network node determines that RS ports that are transmittedfrom the same transmission point, or set of transmission points, arepart of the same set. Other ways of determining the set are possible,and will be described below.

The network node then transmits information indicating the set of RSports to a wireless device, thereby enabling the wireless device toperform joint channel estimation for the RS corresponding to the portsin the set. The set may be associated with an index, and the informationmay then comprise an indication that one or more RS ports are associatedwith the index. Various other ways of representing the information arepossible, and will be described below. The information may betransmitted in an RRC message, or in a DCI format.

Although the described solutions may be implemented in any appropriatetype of telecommunication system supporting any suitable communicationstandards and using any suitable components, particular embodiments ofthe described solutions may be implemented in an LTE network usingdownlink CoMP, such as that shown in FIG. 8, or that illustrated in FIG.14.

The example network may further include any additional elements suitableto support communication between wireless devices or between a wirelessdevice and another communication device (such as a landline telephone).Although the illustrated wireless device may represent a communicationdevice that includes any suitable combination of hardware and/orsoftware, this wireless device may, in particular embodiments, representa device such as the example wireless device 1600 illustrated in greaterdetail by FIG. 16. Similarly, although the illustrated network nodes mayrepresent network nodes that includes any suitable combination ofhardware and/or software, these network nodes may, in particularembodiments, represent devices such as the example network node 1500illustrated in greater detail by FIG. 15.

As shown in FIG. 16, the example wireless device 1600 includesprocessing circuitry 1620, a memory 1630, radio circuitry 1610, and atleast one antenna. The radio circuitry may comprise RF circuitry andbaseband processing circuitry (not shown). In particular embodiments,some or all of the functionality described above as being provided bymobile communication devices or other forms of wireless device may beprovided by the processing circuitry 1620 executing instructions storedon a computer-readable medium, such as the memory 1630 shown in FIG. 16.Alternative embodiments of the wireless device 1600 may includeadditional components beyond those shown in FIG. 16 that may beresponsible for providing certain aspects of the wireless device'sfunctionality, including any of the functionality described above and/orany functionality necessary to support the solution described above.

Still with reference to FIG. 16, some embodiments provide a wirelessdevice 1600 for performing channel estimation. The device comprisesradio circuitry 1610 and processing circuitry 1620. The processingcircuitry 1620 further comprises a channel analyzer 1640 and a channelestimator 1650.

The processing circuitry 1620 is configured to obtain an indication thata set of antenna ports, or antenna port types, share at least onechannel property. In some variants, the processing circuitry 1620 isconfigured to obtain the indication by receiving a message via the radiocircuitry 1610. In other variants, the processing circuitry 1620 isconfigured to determine the set of antenna ports or antenna port typesbased on a rule.

The channel analyzer 1640 is configured to estimate one or more of theshared channel properties based at least on a first reference signaltransmitted from a first antenna port included in the set, or having atype corresponding to one of the types in the set, wherein the firstreference signal is received via the radio circuitry 1610. The channelestimator 1650 is configured to perform channel estimation based on asecond reference signal transmitted from a second antenna port includedin the set, or having a type corresponding to one of the types in theset, wherein the channel estimation is performed using at least theestimated channel properties, and wherein the second reference signal isreceived via the radio circuitry 1610.

In some variants, the channel analyzer 1640 is configured to performjoint estimation of one or more of the shared channel properties, basedon the second reference signal and one or more additional referencesignals received from respective antenna ports included in the set, orhaving a type corresponding to one of the types in the set. The one ormore additional reference signals may comprise the first referencesignal.

In particular embodiments, the channel estimator 1650 is configured togenerate an estimation filter based on the estimated channel properties,and to apply the estimation filter to the second reference signal toobtain a channel estimate. The channel estimator 1650 may be furtherconfigured to apply the estimation filter to at least one otherreference signal received from an antenna port included in the set, orhaving a type corresponding to one of the types in the set, to obtain asecond channel estimate.

In particular embodiments, the processing circuitry 1620 is furtherconfigured to receive an indication of one or more resource block groupsacross which the set of antenna ports or antenna port types share atleast one channel property, and to estimate one or more channelproperties over the indicated set of resource block groups.

In particular embodiments, the processing circuitry 1620 is furtherconfigured to receive an indication of a time interval during which theset of antenna ports or antenna port types share at least one channelproperty, and to estimate one or more channel properties during theindicated time interval, possibly by time-averaging.

As shown in FIG. 15, the example network node 1500 includes processingcircuitry 1520, a memory 1530, radio circuitry 1510, and at least oneantenna. The processing circuitry 1520 may comprise RF circuitry andbaseband processing circuitry (not shown). In particular embodiments,some or all of the functionality described above as being provided by amobile base station, a base station controller, a relay node, a NodeB,an enhanced NodeB, and/or any other type of mobile communications nodemay be provided by the processing circuitry 1520 executing instructionsstored on a computer-readable medium, such as the memory 1530 shown inFIG. 15. Alternative embodiments of the network node 1500 may includeadditional components responsible for providing additionalfunctionality, including any of the functionality identified aboveand/or any functionality necessary to support the solution describedabove.

Referring once more to the block diagram in FIG. 15, some embodimentsprovide a network node 1500 comprising radio circuitry 1510 andprocessing circuitry 1520. The processing circuitry 1520 is configuredto obtain an indication that a set of antenna ports, or antenna porttypes, share at least one channel property. In some variants, theprocessing circuitry 1520 is configured to obtain the indication bydetermining that antenna ports transmitting reference signals that arecode multiplexed using an orthogonal cover code are part of the sameset. The antenna port types correspond to types of reference signalstransmitted by the ports, and comprise one or more of: DMRS, CSI-RS,CRS.

The processing circuitry is further configured to transmit, via theradio circuitry 1510, at least two of the antenna ports in the set, orantenna ports having types comprised in the set, from the sametransmission point.

In some variants, the processing circuitry 1520 is configured totransmit, via the radio circuitry 1510, an indication of the set ofantenna ports, or antenna port types, to at least one wireless deviceserved by the network node. The processing circuitry 1520 may beconfigured to transmit the indication in system information, in an RRCmessage, or in downlink control information.

In some variants, the indication further indicates one or more resourceblock groups across which the set of antenna ports or antenna port typesshare at least one channel property. The processing circuitry 1520 isthen configured to transmit, via radio circuitry 1510, signalscorresponding to at least two of the antenna ports in the set, orantenna ports having types comprised in the set, from the same set oftransmission points over the indicated resource block groups.

In some variants, the indication further indicates a time intervalduring which the set of antenna ports or antenna port types share atleast one channel property, and the processing circuitry 1520 isconfigured to transmit, via radio circuitry 1510, signals correspondingto at least two of the antenna ports in the set, or antenna ports havingtypes comprised in the set, from the same set of transmission pointsduring the indicated time interval.

Modifications and other variants of the described embodiments will cometo mind to one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the embodiments are not to belimited to the specific examples disclosed and that modifications andother variants are intended to be included within the scope of thisdisclosure. Although specific terms may be employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation.

In particular, although various examples disclosed herein refer to auser equipment (UE), this shall not be construed as limiting. It shouldbe appreciated that the methods and concepts apply to wireless devicesin general. Furthermore, wherever examples herein refer to actionsperformed by “the network”, in many implementations such actions will beperformed by a network node, in particular a base station such as aneNB.

Throughout this disclosure, nodes or points in a network are oftenreferred to as being of a certain type, e.g., “macro” or “pico”. Unlessexplicitly stated otherwise, this should not be interpreted as anabsolute quantification of the role of the node/point in the network butrather as a convenient way of discussing the roles of differentnodes/points relative to each other. Thus, a discussion about macro andpicos could for example just as well be applicable to the interactionbetween micros and femtos.

Note that although terminology from 3GPP LTE has been used herein toexemplify the disclosed concepts, this should not be seen as limitingthe scope of this disclosure to only the aforementioned system. Otherwireless systems, including WCDMA, WiMax, UMB and GSM, may also benefitfrom exploiting the ideas covered within this disclosure.

The word “comprise” or “comprising”, as used herein, is intended to beinterpreted as non-limiting, i.e., meaning “consist at least of”.

1-21. (canceled)
 22. A method in a network node, the method comprising:obtaining an indication that a set of antenna ports or reference signalports share at least one channel property; transmitting signalscorresponding to at least two of the antenna ports or reference signalports in the set; and transmitting an indication of the set of antennaports or reference signal ports to at least one wireless device servedby the network node.
 23. The method of claim 22, wherein obtaining theindication comprises determining the set based on a rule.
 24. The methodof claim 22, wherein obtaining the indication comprises obtaining theindication either from another network node or from a storage area,wherein the storage area is internal to the network node or connected toit.
 25. The method of claim 22, wherein the indication is transmitted inan RRC message or in Downlink Control Information.
 26. The method ofclaim 25, wherein the indication further indicates a plurality ofresource blocks or resource block groups across which the set of antennaports or reference signal ports share at least one channel property, andwherein the network node transmits signals corresponding to at least twoof the antenna ports or reference signal ports from the sametransmission point or set of transmission points over the indicatedplurality of resources blocks or resource block groups.
 27. The methodof claim 25, wherein the indication applies until a new indication issent.
 28. The method of claim 22, wherein the signals corresponding toat least two of the antenna ports or reference signal ports in the setare transmitted from a same transmission point or set of transmissionpoints.
 29. The method of claim 22, wherein the indication concerns aset of antenna ports, and the network node transmits the signals fromone transmission point or from two or more transmission points ensuringthat said at least two of the antenna ports or reference signal ports inthe set are co-located.
 30. The method of claim 22, wherein theindication indicates that certain DMRS ports share said at least onechannel property.
 31. The method of claim 22, wherein the indicationindicates at least one CSI-RS resource.
 32. The method of claim 22,wherein the indication indicates that the set of antenna ports orreference signal ports are to be considered co-located.
 33. The methodof claim 22, wherein the channel properties are one or more of:signal-to-noise ratio, delay spread, Doppler spread, received timing,number of significant channel taps.
 34. A network node comprising radiocircuitry and processing circuitry, wherein the processing circuitry isconfigured to: obtain an indication that a set of antenna ports orreference signal ports share at least one channel property; transmit,using the radio circuitry, signals corresponding to at least two of theantenna ports or reference signal ports in the set; and transmit anindication of the set of antenna ports or reference signal ports to atleast one wireless device served by the network node.
 35. The networknode of claim 34, wherein the processing circuitry is configured totransmit the signals corresponding to at least two of the antenna portsor reference signal ports in the set from a same transmission point orset of transmission points.
 36. The network node of claim 34, whereinthe network node controls at least two transmission points and theprocessing circuitry is configured to transmit, using the radiocircuitry, said signals from one of the transmission points or from twoor more transmission points ensuring that said at least two of theantenna ports or reference signal ports are co-located.